In the manufacture of printed circuit boards by the subtractive process, to illustrate a technique widely used at the present time, there is adhered to the surface of a copper-clad laminate one surface of a photopolymerizable film; the other surface of the film is adhered to a film support. The photopolymerizable layer is thereafter exposed to light according to a predetermined pattern, the support is removed and the copper layer exposed by washing away the unexposed areas of the photopolymerizable layer. The exposed copper layer is then etched away by chemical milling.
In the additive process, to describe another useful technique, there is adhered to the surface of a laminate substrate (which may or may not be copper-clad) one surface of a dry, photopolymerizable film; the other surface is adhered to a film support. The photopolymerizable layer is thereafter exposed to a predetermined light pattern, the support is removed and the substrate exposed by washing away the unexposed areas of the photopolymerizable layer. At this point, the substrate is treated with appropriate sensitizers and metallizing solutions, whereby a layer of metal, usually copper, is applied to the substrate.
The dry, photoresist films are typically fairly light in color, and the substrates with which they are used, in the processes illustrated above, are typically copper-clad laminated boards. In this circumstance, it is extremely difficult to inspect the product after exposure to light and prior to removal of the unexposed areas of the layer to insure that the light mask has been properly placed, and to be sure that there are no imperfections or defects in the exposed photopolymerizable layer. It would be extremely useful to have a dry photoresist film which visually indicates where the film has been exposed to light so that detection of defects can readily be made prior to developing the film and etching the copper-clad panels or building up a layer of metal on the exposed substrate.
Liquid photopolymerizable compositions are becoming more and more widely used to replace conventional lacquers, inks, varnishes and paints which all use volatile solvents that contribute substantially to atmospheric pollution. The use of liquid photopolymerizable materials presents a problem, however, particularly when colored systems are employed. Simply stated, the more intense the color, the thinner is the coating that can be used, since the coloration inhibits the penetration of actinic light through the coating. It would be extremely useful to have a liquid photopolymerizable system that is colorless or lightly colored in the unexposed state and which both crosslinks and becomes intensely colored upon exposure to actinic light.